The PBC-6A differs from many other audio compressors in a couple of ways:

No ThresholdThere's no threshold in the PBC-6A. Unlike compressors with threshold controls, the signal passing through the PBC-6A is always being compressed rather than compressing only those parts of the signal that are above a set threashold. This makes the PBC-6A a non-linear, compression amplifier (CA).

Level-Dependent RatioThe PBC uses a feedback configuration to help achieve its function. Along with this, the PBC's detector allows higher output signals to compress more than lower-level ones. With the lack of thresholding, the change in ratio is smooth and gradual from low-level signals to higher-level.

Knee ControlThis allows a transition from lower-to-higher ratios to occur over a smaller range than would "naturally" happen due to the PBC's feedback and detector configuration.

These main features help the PBC-6A achieve a wide-range of compression effects - from delicate to crushed.

Controls

DriveThe PBC's feedback configuration makes it possible to vary both the amount of compression along with some intrinsic make-up gain in one control. This is it. The most compression is attained with the DRIVE control fully clockwise and at a minimum in the fully counterclockwise direction.

KneeThe more clockwise this control, the smaller the change in the input signal that is required to achieve higher compression ratios. Conversely, the more counterclockwise this control, the wider the change in the input signal that is required to achieve higher compression ratios.

AttackAs with many other compressors, this controls how quickly the PBC will react to changes in the input signal. Subjectively, this control allows high frequencies to be more easily emphasized.

ReleaseThis control determines how quickly the PBC will return to its previous gain levels as the signal level decreases.

OutputThis provides a convenient means to help match levels between the PBC and subsequent devices.

BypassThis push button alternatively allows the selection of the signal present at the input jack or the output of the PBC's balanced line driver.

ThickThis alters the PBC's release characteristics in a way that is perceived as "thickening" the sound. This is subtle, but notable in its absence.